This invention relates to thermoplastic copolymers of tetrafluoroethylene (TFE) with improved processibility, and in particular, increased rates of extrusion prior to the onset of melt fracture.
The practical extrusion rate of melt processible or thermoplastic fluoropolymer resins is limited by melt fracture of the polymer. Melt fracture causes roughness and flaws on the surface of the extrudate. It is desirable to increase the useful extrusion rate to increase the output and lower the cost of manufacturing of such resins. One can generally increase the rate at which copolymers of TFE can be extruded, prior to the onset of melt fracture, by decreasing the molecular weight of the copolymer. However, this usually leads to problems with thermal stress cracking in the resulting polymer. Alternatively one can increase the level of comonomer to improve the stress cracking resistance; however, this approach leads to a deterioration in certain electrical properties, such as the dielectric constant and dissipation factor. Fluoroelastomers, such as copolymers of vinylidene fluoride and hexafluoropropylene, can be added to low molecular weight thermoplastic TFE copolymers to improve the thermal stress cracking resistance; however, this also causes a deterioration in certain electrical properties.
Japanese Patent application 52/86442 discloses blends of a TFE/hexafluoropropylene copolymer with fluorine-containing elastomers for improved thermal stress-cracking resistance. The fluorine-containing elastomer is used at a level of from 0.05 to 10 % by weight. Suitable elastomers include HFP/vinylidene fluoride copolymers, TFE/vinylidene fluoride/HFP ternary polymers, TFE/propylene copolymers, TFE/chlorovinyl ether copolymers, TFE/ethylene/propylene ternary copolymers, ethylene/HFP copolymers, TFE/ethylene/HFP ternary copolymers, TFE/ethylene/hexafluoroacetone ternary copolymers, and TFE/hexafluoroacetone copolymers. All other fluorine resins, such as polyvinylidene fluoride, polytetrafluoroethylene, polytetrafluoroethylene wax, and tetrafluoroethylene/fluorovinyl ether, are said to have no effect.
U.S. Pat. No. 4,617,351, Heckel et al., discloses enhancement of the extrusion rate of melt extrudable thermoplastic perfluorocarbon polymers by the addition of up to 1% of a hydrocarbon polymer.
U.S. Pat. No. 4,713,418, Logothetis et al., discloses a blend of 100 parts of a cured fluoroelastomer and 2 to 50 parts of a thermoplastic perfluorocarbon copolymer containing TFE monomer, which is present as generally spherical particles having a particle size of less than 10 microns. The blend has improved tensile strength and tear strength.
U.S. Pat. No. 3,484,503, Magner et al., discloses blends of a copolymer of TFE and perfluoro(methyl vinyl ether) with TFE homopolymer or TFE/hexafluoropropylene copolymer. The TFE/perfluoro(methyl vinyl ether) is the major component of the blend. The blends are characterized by being relatively easy to fabricate and extrude, but also by being resistant to flow at higher temperatures.
U.S. Pat. No. 4,555,543, Effenberger et al., discloses fluoropolymer coating compositions of a fluoroplastic resin dispersion modified by blending with at least 5% of a fluoroelastomer composition, preferably a latex, and films formed therefrom. The fluoroplastic may be polytetrafluoroethylene or fluorinated ethylene propylene copolymer, and the fluoroelastomer may be a copolymer of tetrafluoroethylene with perfluoro(alkyl vinyl ether). Such compositions are useful for manufacture of crack-free coatings.